Single crystal wafer of lithium tantalate

ABSTRACT

The single crystal wafer of lithium tantalate provided by the invention is characterized by the limited range of variation of the value of double refraction which should not exceed ±6×10 -4  so that the value of the double refraction is given by 4.5×10 -3  ±6×10 -4 . When single crystal wafers of lithium tantalate each satisfying the above requirements are used for the manufacture of SAW devices, the devices may have a very small deviation of the SAW sound velocity from the standard value and the range of variation of the SAW sound velocity within a wafer is also very small so that the SAW devices can be manufactured with greatly improved productivity and excellent quality.

This application is a division of application Ser. No. 810,748, filed Dec. 19, 1985, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a single crystal wafer of lithium tantalate or, more particularly, to a single crystal wafer of lithium tantalate suitable as a base material for the preparation of SAW devices having well controlled performance and quality.

As is known, single crystals of lithium tantalate, lithium niobate, quartz and the like are useful as a ferroelectric material and piezoelectric material and, for example, so-called SAW devices are practically manufactured from these single crystal materials. Among these single crystal materials, in particular, lithium tantalate has a relatively high electromechanical coupling coefficient and small temperature dependency of various characteristic properties so that single crystals of lithium tantalate are useful as a material of VCR resonators and PIF filters of television sets as well as some devices for high-frequency communication instruments.

Single crystals of lithium tantalate are usually prepared by the Czochralski method in which the single crystal is grown on a seed crystal pulled up from the melt of an oxide mixture having a composition corresponding to LiTaO₃. The single crystal boule as grown is first subjected to a poling treatment to align the ferroelectric domains into a single-domain body which is then mechanically worked into a cylindrical form and sliced into wafers having a definite crystallographic orientation. The wafer is lapped and polished on the surface before it is used as a base plate of various kinds of devices.

For example, SAW devices are manufactured by cutting such a wafer polished and provided with an electrode of, mostly, aluminum on the surface into small square or rectangular chips. The number of the chips obtained by cutting a single wafer may be only one or may be plural depending on the diameter of the wafer and the dimensions of the desired devices. For example, it is not rare that 200 or more of devices can be manufactured by cutting a single wafer having a diameter of 3 inches. It is of course that the productivity of this process can be increased as the number of the devices taken from a single wafer is increased so that single crystal wafers having larger and larger diameters are desired as a trend toward 3 inches, 4 inches or larger diameters from the 2-inch wafers quite acceptable several years ago.

Such a recent trend of manufacturing a larger and larger number of devices from a single wafer of lithium tantalate single crystals is accompanied by a problem that, along with the increased requirement for uniform and well-controlled performance and quality of the SAW devices obtained from a single wafer, some of the characteristic parameters of the single crystal wafer vary considerably from portion to portion of a wafer and the variation of the values is more remarkable when the diameter of the wafer is larger. The most serious problem in this regard is the variation in the values of the optical properties or double refraction which is one of the most important characteristic parameters in the single crystal wafers of lithium tantalate for manufacturing SAW devices.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide single crystal wafers of lithium tantalate used for the preparation of SAW devices free from the above mentioned disadvantages and problems in the prior art. The inventors have arrived at a conclusion after an extensive study on the relationship between the value of double refraction in the wafer as an optical property thereof and the sound velocity through an SAW device that such an object can be achieved when the double refraction of the wafer has a specific value with the variation thereof in a wafer being within a certain limit. Namely, the variation of the value of double refraction in a single wafer of lithium tantalate should be within ±6×10⁻⁴ since otherwise the sound velocity in the SAW devices prepared from the wafer may have a variation reaching ±0.15% so that the SAW devices cannot have uniformly controlled quality and performance.

Thus, the single crystal wafer of lithium tantalate according to the invention completed on the base of the above discovery is a wafer of which the variation in the value of the double refraction determined at a wave length of 632.8 nm and at a temperature of 20° C. does not exceed ±6×10⁻⁴ or, preferably, ±3×10⁻⁴ and the value of the double refraction determined at the same wave length and at the same temperature as above is 4.5×10⁻³ ±6×10⁻⁴ or, preferably, 4.5×10⁻³ ±3×10⁻⁴.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of the optical system for the determination of the double refraction of a wafer.

FIG. 2 illustrates a digital-formed filter prepared by patterning on a single crystal wafer of lithium tantalate in such a manner that the direction of propagation of SAW should be in parallel with the direction of 112°Y.

FIG. 3 is a block diagram of the electric circuit for the determination of the sound velocity of SAW in an SAW device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the single crystal wafer of lithium tantalate according to the invention is illustrated by way of examples and with reference to the accompanying drawing.

Several single crystal wafers of lithium tantalate were prepared each having a value of the double refraction and a range of variation of the double refraction within the wafer as shown in Table 1 below. These data tabulated in Table 1 were obtained by the measurements at 20° C. using a wave length of 632.8 nm in an optical system illustrated in FIG. 1.

Each of the wafers was provided on the surface with the digital-formed filters as shown in FIG. 2 by patterning in such an orientation that the direction of propagation of SAW was in parallel with the direction of 112°Y and the sound velocity V of SAW was measured by use of the measurement system of the block diagram illustrated in FIG. 3. The phase difference between the input and output signals was determined with the frequency of the signal generator under automatic scanning and the sound velocity V was calculated from the following equation: ##EQU1## in which V is the sound velocity, L is the distance between the digital-formed electrodes, dφ is the change in the phase and dW is the change in the frequency. V was determined as V=L·ΔW/20 by means of CPU to automatically determine the frequency change W of which the value of dφ was behind by 20 times of the wavelength. The frequency used for the measurement was varied in the range from 95 MHz to 105 MHz.

The results were as shown in Table 1, according to which, among the 8 experiments, quite satisfactory results were obtained in Experiments No. 2, No. 4, No. 5 and No. 6 since both of the maximum deviation of the sound velocity of SAW from the standard value and the range of variation of the sound velocity of SAW within a wafer were small. The data in Table 1 were all obtained at 20° C.

                  TABLE 1                                                          ______________________________________                                                                             Range of                                                 Range of              variation                                                variation Maximum de- in percent                                               of double viation in percent of                                                                      of SAW                                           Double  refraction                                                                               SAW sound   sound ve-                                  Exper-                                                                               refrac- within a  velocity    locity                                     iment tion, ×                                                                          wafer,    from stan-  within a                                   No.   10.sup.-3                                                                              × 10.sup.-3                                                                        dard value  wafer                                      ______________________________________                                         1     5.2     ±0.6   0.17        ±0.08                                   2     5.0     ±0.1   0.08        ±0.01                                   3     4.8     ±0.8   0.14        ±0.10                                   4     4.6     ±0.2   0.04        ±0.02                                   5     4.4     ±0.5   0.08        ±0.06                                   6     4.2     ±0.3   0.08        ±0.04                                   7     4.0     ±0.7   0.16        ±0.09                                   8     3.8     ±0.3   0.13        ±0.04                                   ______________________________________                                     

What is claimed is:
 1. A method for selecting high quality single crystal wafers of lithium tantalate for use in the manufacture of SAW devices comprising determining the double refraction value and the value of the variation of double refraction of the single crystal wafers at a wavelength of 632.8 nm and at a temperature of 20° C. and selecting the high quality single crystal wafers which have a double refraction value of 4.5×10⁻³ ±6×10⁻⁴ and in which the value of the variation of double refraction does not exceed ±6×10⁻⁴. 